1. Field of the Invention
The present invention relates to electric transformers for supplying the power circuits of microwave ovens with electric power. These transformers include a primary electric winding, in the form of a coil, the two ends of the winding wire being connected electrically to tags for connection to the mains supply network. A secondary high voltage electric winding is formed of a coiled electric conductor a first end of which is connected electrically to the transformer magnetic circuit and to the ground of the microwave oven, and the other end of which is connected electrically to a high voltage output tag for connection to the high voltage electric elements of the microwave oven for supplying them with power. The transformer further includes a secondary electric heating winding formed of a coiled conductor, with a small number of turns, the two ends of the heating conductor being provided with tags for connection to the filament heating electric circuit of the magnetron of the microwave oven. The magnetic circuit of the transformer includes two lateral columns and a central column all three connected together by end cross pieces. An intermediate magnetic shunt partially loops the magnetic circuit between the lateral columns and the central column, deflecting a part of the magnetic flow between the primary electric winding and the secondary high voltage electric winding.
The power usually delivered by such a transformer ranges between 200 and 1500 W, so that it is a question of low power transformers. The primary supply voltage is generally 110-240 V. The high voltage output voltage is generally about 2300 V and must have a special waveform which the magnetic circuit with a special magnetic shunt confers thereon.
It is thus indispensable to provide very efficient electric insulation, able to withstand a voltage of about 10 kV without breakdown between the different electric windings and between the electric windings and the magnetic circuit.
2. Description of the Prior Art
The electric insulation is generally provided by insulating foils folded and wrapped adequately about the windings and between the windings and the magnetic circuit. This insulating technique is described for example in the patent U.S. Pat. No. 2,858,514. Thus, all the transformers for microwave ovens at present on the market include such a type of electric insulation. The operations for providing such insulations are particularly time consuming and tedious, and substantially increase the production cost. These operations are necessarily manual and it is practically impossible to automate the production of such transformers.
The German model of Utility DE-U No. 8 633 338 describes a transformer for microwave ovens in which the electric windings are formed on carcases made from an insulating material. Such a technique is not sufficient for providing complete insulation of the electric circuit with respect to the magnetic circuit and requires a complementary winding of a ribbon on the electric winding on the carcases. It can be further noted that the magnetic circuit described in this document has a complex shape, with cut outs which reduce the magnetic circuit section in certain zones and disturb the flow of the magnetic flux.